This invention relates to a filter for capturing fine particles consisting mainly of carbon in exhaust gases exhausted from combustion engines such as diesel engines and burning the particles to purify the exhaust gases.
A filter for purifying exhaust gases from combustion engines such as diesel engines by removing fine particles consisting mainly of carbon included in the exhaust gases has been known as disclosed, for example, in U.S. Pat. No. 4,364,761 comprising a ceramic honeycomb structure having alternately closed through-apertures. In such a filter, as the fine particles are accumulated in partition walls of the honeycomb structure, pressure losses become large to lower engine performance. Therefore, it is necessary to burn the accumulated fine particles so as to recover the filtering performance.
In conventional honeycomb structures, the partition walls thereof for filtering are usually formed by extruding so that the thicknesses of the walls, diameters of pores and porosity of the walls are substantially uniform throughout the structure. The fine particles are therefore accumulated uniformly form inlets to outlets of the partition walls of the honeycomb structures or increasing at the outlet ends.
When the particles are burned to recover the filter, the fine particle layers are progressively burned from the inlet side to the outlet side to restore the filter. In the conventional honeycomb filters adapted to accumulate fine particles thereon, burning heat produced on the inlet side transmits to the outlet side, and further, the temperature of partition walls on the outlet side rapidly rises because of the burning heat of the fine particles on the outlet sides, so that the ceramic filter may melt or cracks may ultimately occur due to thermal shock in such a rapid temperature rise.
In order to eliminate such a disadvantage, porous ceramic homeycomb structures are so formed by a corrugate method that the porosity of paritition walls decreases from the inlet side to the outlet side as disclosed in Japanese Patent Application Laid-open No. 61-129,017. In order to produce such filters having the uneven porosity, however, ceramic green sheets having different porosities must be prepared. What is worse still, such ceramic green sheets of the different porosities make the production of honeycomb structures difficult due to different contraction rates dependent upon their porosities when formed green sheets are fired.
Moreover, in order to produce filters having various porosities, various ceramic green sheets must be prepared because the porosities are determined by ceramic green sheets.